U.S. Prepares for Solo Strike On Syria After Britain Balks

WASHINGTON—The Obama administration laid the groundwork for unilateral military action in Syria, a shift officials said reflected the U.K.'s abrupt decision not to participate and concerns that President Bashar al-Assad was using the delayed Western response to disperse his military assets.

The push for a quick international strike to punish Syria for what the U.S. said was a chemical-weapons attack appeared in disarray on Thursday,...